I would like to suggest the 7,7 cm WD Schlepper 50 PS. Funny name, what is it? Oh thats simple its a classical 1920 fully tracked tractor with some armor put on and a 7,7 cm FK 96 L/27 put on a 30° platform with gunshield on it. Crew of 2 1 build and put into service. Funny story, but similar to the Zis 30.
The Wehrmacht gave on the 08.10.1926 an order to Krupp AG to construct an SPG for a 7,5 cm Infantry gun. It was an civil Fully tracked vehicle to be used and a Gun with shield of 8mm with 15° traverse to each side as well as -10 and +40° elevation. The Hull armor 10mm and a max speed of 12 km/H. ammo was at the back in 5mm armored boxes.
The gun is the 7,7 cm FK 96 with about the same projectiles as the later 7,5 cm K 37 L/24 (the FK 96 were later also rechamered for 7,5 mm ammo which is based on the 7,7 cm ammo)
One He grenade was called Feldgranate 96 with a weight of 6,8 kg and 190g of TNT 465m/s and some ap performance.
The K.Gr. 15 with 7,1 kg 380 g amatol or 270g Pieric acid (in game x1,1= 297g, IRL x1,2=324g) and 465m/s
The L F.K.Gr. with 7,18 kg 930g amatol or 700g Pieric acid( ig x1,1=770g, IRL x1,2=840g) or 820g amatol with 75g Pieric acid. 465m/s
There was also AP grenades, just stating the 2 gameplay wise interesting:
The K.Gr. 15 m.P m.L with 7 kg, 230g filler fired at 465m/s, a tracer and 52mm at 10m
The K.Gr. 15 m.P. with 6,85 kg, 230g fille 465m/s, no tracer and 51mm at10m
The name is Kanonen Granate 15 mit Panzerkopf,
m.L. mit Leuchtspur
Bo.Pr Bohrgeschoß Preßstrahlform
10mm Hull armor all round
50 HP at 850 r/min
7,7 cm FK 96
15° left and right each
around 10-40 rounds in 5mm ammo box at the back
(Its so old its hard to find reliable info on it)
The far right black round with read tip is the Aphe K.Gr. 15 m.P.
Sources: Thomas L.Jentz, Hilary Louis Doyle - Panzer Tracts No.7-1, Panzerjäger (3,7cm Tak bis Pz.Sfl.Ic) Entwicklung und Einsatz von 1927 bis 1941
Walter J. Spielberger - Die RAD- und Vollkettenzugmaschinen des Deutschen Heeres 1870-1945